SVP abstracts – Origin of aquatic reptiles?

Sobral and Schoch 2019
bring us news on a taxon at the genesis of aquatic reptiles.

I presume that means
no sea turtles, no marine iguanas, no mosasaurs, no sea crocs, no penguins. If so, the LRT already provides a long list of diapsid taxa at the base of the Enaliosauria (Fig. 1; including mesosaurs, ichthyosaurs, thalattosaurs, placodonts, pachypleurosaurs and plesiosaurs) along with other basal aquatic marine younginiforms (Fig. 2), a monophyletic clade distinct from terrestrial younginiforms that gave rise to protorosaurs and archosauriforms.

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

Figure 1. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

From the abstract:
“The Middle Triassic was a time of major changes in terrestrial tetrapod faunas, but the fossil record of this interval is largely obscure.”

Why do paleontologists always paint themselves into a corner like this? To make their discoveries more newsworthy?

“This is unfortunate, since many modern groups originated or diversified during this time. However, recent excavations in the upper Middle Triassic of Germany have revealed several new taxa, most of which are much smaller than those found in other tetrapod-bearing basins of similar age.”

Here’s Galesphyris (Fig. 2) at the base of the aquatic younginiforms in the LRT.

Figure 3. Spinoaequalis and descendant marine younginiformes.

Figure 3. Spinoaequalis and descendant marine younginiformes. These give rise to plesiosaurs, placodonts, mesosaurs, ichthyosaurs and thalattosuchians. Click to enlarge.

Sobral and Shoch continue:
“Here, we report a new taxon from the Vellberg limestone quarry comprised of skull bones distinct from other diapsids from this locality. It is diagnosed by a long maxilla with a far posteriorly reaching tooth row; a long and stout ventral process of the postfrontal; exclusion of the postorbital from the lower temporal fenestra due to a contact between the anteroventral process of the squamosal and the dorsal process of the jugal; and a tall quadrate + quadratojugal complex.”

“Some anatomical aspects of the new taxon are similar to stem diapsids such as Elachistosuchus huenei from similar deposits of Northern Germany and of uncertain phylogenetic affinity.”

In the LRT Elachistosuchus (Fig. 3) nests certainly between proterosuchids and choristoderes (Fig. 4). Neither are related to aquatic younginiforms.

Figure 1. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Figure 3. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Figure 4. This is where Elachistosuchus nests, next to BPI 2871, at the base of the Choristodera.

Figure 4. This is where Elachistosuchus nests, next to BPI 2871, at the base of the Choristodera.

“A phylogenetic analysis retrieved both taxa in an “ichthyosauromorph” clade, included in an almost exclusively aquatic group. The new taxon, Hupehsuchus, and Elachistosuchus appear as successive sister-taxa to Ichthyopterygia.”

This is not supported by the LRT where Hupehsuchus (Fig. 5) and Elachistosuchus (Fig. 3) are not related  to one another. The outgroups to the Ichthyopterygia (Fig. 1) are the Thalattosuchia, Mesosauria and basal Sauropterygia (pachypleurosaurs).

Figure 2. Basal Ichthyosauria, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and Thaisaurus

Figure 5. Basal Ichthyosauria in the LRT, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and ThesaurusFigure 2. Basal Ichthyosauria, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and Thaisaurus

“It is interesting to note that many of the autapomorphic characters of the new taxon pertain to elements related to the lower temporal fenestra. In particular, the contact between the jugal and squamosal is unusual, but is also found in sauropterygians, saurosphargids, Hupehsuchus, and Wumengosaurus, as well as in rhynchocephalians.”

Oh, why did they have to add rhynchocephalians? They were doing so well! Readers beware, convergence is rampant (= everywhere) in the Reptilia. Don’t rely on one, two or a dozen traits. If you do, you’ll be pulling a Larry Martin. Only rely on the last common ancestor in a valid cladogram to determine relationships.

“Derived ichthyosaurs show the typical jugal-quadratojugal contact, but via an unusual dorsal contact between the two. The jugal–squamosal contact may thus represent a transitional state to the anatomy observed in later ichthyosaurs, reinforcing the interpretation of the ‘ventral cheek embayment’ of basal ‘euryapsids’ as a ventrally open lower temporal fenestra.”

“Thus, the new taxon has implications for the origin of secondarily aquatic groups, and therefore also paleobiogeographic significance. The appearance of placodontians has been traced to central Europe, but ichthyopterygians are believed to have originated in the Eastern Tethys. The new taxon indicates that the earliest evolutionary history of these groups may have occurred in the Western Tethys, associated with the Germanic Basin. This new material emphasizes the importance of sampling small-bodied taxa in the understanding of reptile evolution.”

The Lower Keuper is Carnian, early Late Triassic. Galesphyris is older. It comes from the Late Permian, perhaps representing an early Early Permian genesis.


References
Sobral G and Shoch R 2019. A small diapsid from the Lower Keuper of Germany and the origin of aquatic reptiles. Journal of Vertebrate Paleontology abstracts.

The origin of marine crocs re-revised

Revised August 09, 2019
with the addition of the Dyoplax skull (Fig. 7b) recently downloaded from Maisch et al. 2013

Also revised March 31, 2019, 
with a repaired nesting for Fruitachampsa (Figs. 4,5) a sister to Protosuchus in the LRT (subset Fig. 3), not a sister to marine crocs.

Figure 2. Several Jurassic sea crocs, apparently derived from Late Triassic Dyoplax.

Figure 1. Several Jurassic sea crocs, apparently derived from Late Triassic Dyoplax.

Dr. Andrea Cau 2019
recently revised the affinities of the extinct marine crocs (Figs. 1,2). Here (Fig. 3), with more outgroup taxa, the affinities of the outgroups are more refined by adding taxa omitted by Cau. The in-group marine croc clade of Cau 2019 continue as is untested.

Figure 1. Reduced from Cau 2019 showing Fruitachampsa as the proximal outgroup for marine and river crocs.

Figure 2. Reduced from Cau 2019 showing Fruitachampsa as the proximal outgroup for marine and river crocs. The outgroup Postosuchus is not related to Crocodylomorpha in the LRT.

As we learned earlier
choosing outgroup taxa is not as scientific as letting a wide gamut phylogenetic analysis, like the large reptile tree (LRT, 1549 taxa, subset Fig. 3), choose outgroup taxa for you. 

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Strangely,
the proximal outgroup taxon for marine crocs recovered by Dr. Cau (Fig. 2) was tiny Fruitachampsa (Figs. 3, 4), a small, gracile Late Jurassic biped sprinter that nests with Protosuchus in LRT and Cau’s cladogram. Fruitachampsa would seem to have few traits in common with river and marine crocs. Dyoplax has more.

Figure 1. Fruitachampsa reconstructed. Note the homologies with Scleromochlus.

Figure 4. Fruitachampsa reconstructed. Note the many homologies with Scleromochlus and the few with marine crocs.

Cau did not include Dyoplax
in his cladogram.

Figure 5. Fruitachampsa skull. The vomers are missing. The chonae are conjoined medially, contra Clark 2011.

Figure 5. Fruitachampsa skull. The vomers are missing. The chonae are conjoined medially, contra Clark 2011.

While we’re on the subject of Fruitachampsa,
it had an enormous notch for a mandibular fang, much larger than necessary. The medial choana is similar to sister taxa.

Despite appearances
Dyoplax (Fig. 7) is not considered a crocodylomorph, let alone an outgroup to marine crocs, as we learned earlier here.

Figure 7. Dyoplax arenaceus Fraas 1867 is a mold fossil recently considered to be a sphenosuchian crocodylomorph. Here it nests as a basal metriorhynchid (sea crocodile) in the Late Triassic.

Figure 7. Dyoplax arenaceus Fraas 1867 is a mold fossil recently considered to be a sphenosuchian crocodylomorph. Here it nests as a basal metriorhynchid (sea crocodile) in the Late Triassic.

Figure 7b. Added 08/09/19 from Maisch et al. 2013. DGS sutures do not match sutures found by Maisch et al. Hypothetical missing parts based on phylogenetic bracketing ghosted on.

Figure 7b. Added 08/09/19 from Maisch et al. 2013. DGS sutures do not match sutures found by Maisch et al. Hypothetical missing parts based on phylogenetic bracketing ghosted on.

In the LRT
(subset Fig. 3) Dyopolax (Figs. 7, 7b) is the outgroup taxon to marine crocs while Dibrothosuchus (Fig. 8) is basal to this clade + river crocs.  

Figure 8. Dibothrosuchus nests basal to all later quadrupedal crocs, including marine crocs, in the LRT.

Figure 8. Dibothrosuchus nests basal to all later quadrupedal crocs, including marine crocs, in the LRT. The hind limbs are not known. Phylogenetic bracketing suggests shorter legs are more likely.

Once again,
a wide gamut phylogenetic analysis is key to recovering interrelationships.


References
Cau A 2019. A revision of the diagnosis and affinities of the metriorhynchoids (Crocodylomorpha, Thalattosuchia) from the Rosso Ammonitico Veronese Formation (Jurassic of Italy) using specimen-level analyses. PeerJ, DOI 10.7717/peerj.7364
Clark JM 2011. A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado. Zoological Journal of the Linnean Society, 2011, 163, S152–S172. doi: 10.1111/j.1096-3642.2011.00719.x

 

Was Mesosaurus fully aquatic?

A new paper by Demarco, Meneghel, Laurin and Piñeiro 2018
asks, Was Mesosaurus (Fig. 1) a fully aquatic reptile? The authors report, “Mesosaurs are widely thought to represent the earliest fully aquatic amniotes,” but conclude, “more mature individuals might hypothetically have spent time on land. In this study, we have found that the variation of the vertebral centrum length along the axial skeleton of Mesosaurus tenuidens fits better with a semi-aquatic morphometric pattern, as shown by comparisons with other extinct and extant taxa.”

Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen.

Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen. None of these taxa appear to be fully aquatic, but related thalatttosaurs and ichthyosaurs definitely were.

The authors report methods
“We measured the centrum length for each available vertebra in the mesosaur skeletons. All measurements were taken on digital images.” They also looked at Claudiosaurus and Thadeosaurus (Fig. 1), but did not conduct a phylogenetic analysis that included these and other closest sisters to Mesosaurus in the large reptile tree (LRT, 1263 taxa). For comparison, the authors looked at the unrelated vertebral profiles of Cotylorhynchus, Casea, Varanus and Varanops

All of the ancestors to Mesosaurus in the LRT
kept four functioning legs, so terrestrial locomotion remained within their abilities. That seems pretty clear. At Anarosaurus (Fig. 1) the Sauropterygia split off with Pachypleurosaurus and Diandongosaurus at the base. At Brazilosaurus the Thalattosauria + Ichthyosaurus split off with Wumengosaurus (Middle Triassic)  and Serpianosaurus (Middle Triassic) at the base. That means taxa from Galephyrus to Wumengosaurus had their genesis prior to the Early Permian, in the Late Carboniferous. That gives time enough for basal ichthyosaurs, like Grippia, to appear in the Early Triassic. This is a prediction that can be tested and confirmed with new discoveries in the Late Carboniferous.

Note that basal marine younginiform diapsids
are basal to the clade Enaliosauria, which includes mesosaurs, sauropterygians, thalattosaurs and ichthyosaurs in the LRT. Mesosaurs were not basal anapsids (contra Demarco et al. 2018 and all prior authors dealing with mesosaurs).

The authors report,
“The evidence suggests thatMesosaurus may have been slightly amphibious rather than strictly aquatic, at least when it attained a large size and an advanced ontogenetic age, though it is impossible to determine how much time was spent on land and what kind of activity was performed there. Thus, it is impossible to know if mesosaurids came onto land only to bask, like seals or crocodiles, or if they were a bit more agile.”

Since mesosaurs still had limbs, hands and feet,
we can imagine/surmise that they were able to crawl about on land. Based on their proximity to thalattosaurs and ichthyosaurs and the derivation from basal sauropterygians, they were aquatic as well.

It is noteworthy
that sauropterygians and ichthyosaurs experienced live birth. So, it is not surprising that mesosaurs, nesting between them, were also viviparous (Piñeiro et al. 2012).

Interesting
that mesosaurs despite their derived nesting, predate their late-surviving phylogenetic ancestors. This demonstrates the incompleteness of the fossil record and the likelihood of finding phylogenetic ancestors in earlier strata, which happens all the time

References
Demarco PN, Meneghel M,  Laurin M and Piñeiro G 2018. Was Mesosaurus a fully aquatic reptile? Frontiers in Ecology and Evolutiion 6:109. doi: 10.3389/fevo.2018.00109
Piñeiro G, Ferigolo J, Meneghel, M and  Laurin M 2012. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology. 24 (6): 620–630. doi:10.1080/08912963.2012.662230

Paludidraco and Cymatosaurus in the LRT

It’s been awhile since we looked at anything wet.
A new robust-ribbed sauropterygian, Paludidraco ( Fig. 1, Middle Triassic) does indeed share many traits with Simosaurus, as described by Chaves et al. 2018.

A welcome confirmation!
Due to its tiny dentition, Paludidraco was originally considered a likely filter feeder, distinct from related, long-toothed nothosaurs and plesiosaurs. Simosaurus also has relatively tiny teeth, but on a larger skull and fewer in number. That’s evolution at work!

Isn’t it great to see these two related taxa together? Doesn’t it make compare and contrast so much easier? See the evolution of the human ear bones from primitive jaw bones illustration here for another great example of comparative anatomy.

Figure 1. Simosaurus compared to Paludidraco.

Figure 1. Simosaurus compared to Paludidraco. Isn’t it great to see these two related taxa together? Doesn’t it make compare and contrast so much easier? 

Chaves et al. 2018 provided
a cladogram of marine reptile relationships (Fig. 2). Most of these taxa are also included in the large reptile tree ( LRT, 1261 taxa, subsets Figs. 3, 4), which includes many times more taxa and more marine reptiles. Missing from the Chavez team cladogram (Fig. 2) is the genus/taxon Anningsaura, which links nothosaurs to pistosaurs + plesiosaurs in the LRT. The Chaves et al. cladogram, nests Cymatosaurus (Fig. 4) and Corosaurus basal to Pistosaurus + plesiosaurs.

Figure 2. Paludidraco cladogram with arrows showing how taxa nest in the LRT. Taxon exclusion is the problem here.

Figure 2. Paludidraco cladogram from Chaves et al. 2018 with arrows showing how taxa nest in the LRT. Taxon exclusion is the problem here. See figure 3.

The Chaves et al. 2018 cladogram
(Fig. 2) excludes many pertinent taxa, so much so that important interrelationships were missed, based on the authority of the LRT (Fig. 3), which minimizes taxon exclusion due to its wider gamut of taxon inclusion. Several taxa in the Chaves et all cladogram would shift positions when tested with more taxa (arrows in Fig. 2) as the LRT shows (Fig. 3).

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria). Paludidraco was not added when this graphic was created, but has since been added. Sharp-eyed readers will see Vancleavea here.

Cymatosaurus
had to be added to the LRT (Fig. 4) to test it fairly against the Chavez team cladogram (Fig. 2). Only the skull is known (AFAIK) from three different species.

FIgure 4. The addition of Cymatosaurus is more of an insertion, that changes nothing else in the tree topology. Here it nests on the nothosaur side of Simosaurus.

FIgure 4. The addition of Cymatosaurus is more of an insertion, that changes nothing else in the tree topology. Here it nests on the nothosaur side of Simosaurus, not close to plesiosaurs.

Despite the many offshoot traits
found in Anningsaura, the rest of its traits nest it firmly at the base of the pistosaurs + plesiosaurs, where Chaves et al. nests Cymatosaurus. In the LRT Cymatosaurus nests close to Paludidraco, but more on the nothosaur side than the plesiosaur side.

References
Chaves C de M, Ortega F and Pérez‐García A 2018. New highly pachyostotic nothosauroid interpreted as a filter-feeding Triassic marine reptile. Biology Letters. 14 (8): 20180130.
Maisch MW 2014. A well preserved skull of Cymatosaurus (Reptilia: Sauropterygia) from the uppermost Buntsandstein (Middle Triassic) of Germany. Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen272 (2): 213–224.

wiki/Paludidraco

Another long-necked embryo tritosaur: Li et al. in press

This appears to be
yet another Tanystropheus-like and Dinocephalosaurus-like taxon, yet not closely related to either. Earlier we looked at another similar embryo, still within its mother.

Li, Rieppel and Fraser in press (2017)
bring us a new curled up (as if in an egg, but without a shell) embryo from the Guanling Formation (Anisian), Yunnan province, China (Figs. 1, 2). The specimen is unnamed and not numbered. It appears to combine the large head and eyes of langobardisaurs with the short limbs and many cervical vertebrae of Dinocephalosaurus. Please remember, in this clade, juveniles do not have a short rostrum and large eyes unless their parents also had these traits.

Figure 1. The unnamed and not numbered Triassic embryo Li et al. assign to a new species close to Dinocephalosaurus.

Figure 1. The unnamed and not numbered Triassic embryo Li et al. assign to a new species close to Dinocephalosaurus. At 72 dpi monitor resolution, this image is 2.5x life size. Here bones are colorized, something Li et al. could have done, but avoided. I’m happy to report that the line drawing was traced by Li et al. on their own photo. The two are a perfect match.

Unfortunately
Li et al. have no idea what they’re dealing with phylogenetically. They relied on old invalidated hypotheses of relationships. They report the specimen:

  1.  is a marine protorosaur and an archosauromorph – actually it is a marine tritosaur lepidosaur. Taxon exclusion and traditional bias hampered the opinion of Li et al. They did not perform a phylogenetic analysis.
  2. is closely related to Dinocephalosaurus – actually it is more closely related to the much smaller, but longer-legged Pectodens (Figs. 4, 5). In the large reptile tree (LRT, 1036 taxa) 8 steps are added when the embryo is force-nested with Dinocephalosaurus. The embryo is distinct enough that the new specimen deserves a new genus.
  3. confirms viviparity – probably not (but see below). The specimen is confined within an elliptical shape (Fig. 1), as if bound by an eggshell or membrane, which was not preserved. Perhaps, as in pterosaurs and many other lepidosaurs, the embryo was held within the mother’s body until just before hatching, within the thinnest of egg shells and/or membranes.
  4. is too immature to describe it as a new taxon – not so. Tritosaur lepidosaurs (from Huehuecuetzpalli to Pterodaustro) develop isometrically. Thus, full-term embryos and hatchlings have adult proportions.
Figure 2. The specimen from figure 1 unrolled for clarity. This specimen most closely resembles the basal langobardisaur, Pectodens, not Dinocephalosaurus. Remember, tritosaurs develop isometrically. Embryos closely resemble adults. That's why three scale bars are included.

Figure 2. The specimen from figure 1 unrolled for clarity. This specimen most closely resembles the basal langobardisaur, Pectodens, not Dinocephalosaurus. Remember, tritosaurs develop isometrically. Embryos closely resemble adults. That’s why three scale bars are included. This specimen has feeble limbs but a strong swimming tail, distinct from that of Dinocephalosaurus.

Li et al. report
“In the fossil record only oviparity and viviparity can be distinguished, Ovoviviparity of different intermediate stages, which is often observed in modern squamates would then be referred to the category of viviparity, whatever the stages of maturity and nutritional patterns are.” Yes, they correctly report ovoviviparity in squamates, which are the closet living relatives of tritosaur lepidosaurs. That’s exactly what we have here.

Figure 1. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Figure 3. The new Dinocephalosaurus has traits the holotype does not, like a longer neck with more vertebrae, a robust tail with deep chevrons and a distinct foot morphology with an elongate pedal digit 4.

Li et al. report,
“[The] skeleton is preserved tightly curled so as to produce an almost perfect circular outline, which is strongly indicative of an embryonic position constrained by an uncalcified egg membrane.”

Figure 2. Pectodens skull traced using DGS techniques and reassembled below.

Figure 4. Pectodens skull traced using DGS techniques and reassembled below. No sclerotic ring here. 

Distinct from Pectodens the new genus embryo has:

  1. 24 cervicals
  2. 29 dorsals
  3. 2 sacrals
  4. and about 64 caudals
Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Figure 5. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017. The skull shown here is the original reconstruction. Compare it to figure 4.

Li et al overlooked:

  1. strap-like coracoids, strip-like clavicle and T-shaped interclavicle
  2. scattered manual elements
  3. pelvic girdle
  4. ectopterygoid, jugal, articular, angular, surangular

Li et al. report:
“The fewer cervical vertebrae (24 as opposed to 33 (based on an undescribed specimen kept in the IVPP)), and the presence of sclerotic plates are features inconsistent with Dinocephalosaurus.This embryo therefore documents the presence of at least one additional dinocephalosaur-like species swimming in the Middle Triassic of the Eastern Tethys Sea.

“Scleral ossicles have previously not been described in any protorosaur.”
– but they are common in tritosaur lepidosaurs, like pterosaurs.

Figure 6. Pectodens adult compared to today's embryo and its 8x larger adult counterpart after isometric scaling.

Figure 6. Pectodens adult compared to today’s embryo and its 8x larger adult counterpart after isometric scaling. Looks more like Pectodens than Dinocephalosaurus, doesn’t it? See taxon inclusion WORKS! Sclerotic rings were omitted here to show skull bones. The ring would have had a smaller diameter if if were surrounding a sphere, rather than crushed flat. 

A word to traditional paleontologists:
Don’t keep digging yourself deeper into invalidated hypotheses and paradigms. Use the LRT, at least for options.

Don’t give up on naming embryos
and adding them to phylogenetic analysis, especially if they are tritosaur lepidosaurs. Hatchlings nest with adults so you can used hatchlings in analysis.

Don’t avoid creating reconstructions.
That’s a great way to discover little splinters of bone, like clavicles and coracoids, that would have been otherwise overlooked.

The LRT is here for you.
BETTER to check this catalog prior to submission rather than have your work criticized for being unaware of the latest discoveries or overlooking pertinent taxa AFTER publication.

References
Li C, Rieppel O, Fraser N C, in press. Viviparity in a Triassic marine archosauromorph reptile. Vertebrata PalAsiatica, online here.

Atopodentatus and Claudiosaurus compared

Figure 1. Click to enlarge. Atopodentatus and Claudiosaurus (ghosted) and the small one above Atopo's shoulder to scale. Sometimes it just helps to compare sister taxa to see how far, and in what direction, they have come.

Figure 1. Click to enlarge. Atopodentatus and Claudiosaurus (ghosted) and the small one above Atopo’s shoulder to scale. Sometimes it just helps to compare sister taxa to see how far, and in what direction, they have come.

Earlier we looked at the latest Triassic marvel, Atopodentatus unicus. Today there’s a reconstruction (Fig. 1) alongside a more familiar and pleisomorphic Claudiosaurus for comparison.

Atopodentatus was bigger overall with a relatively larger and more robust torso and tail and a weaker shoulder girdle and pelvis, making it a slower swimmer, but more of a tail swimmer. The humerus was larger, but the antebrachium was smaller, so the forelimbs were likely acting as props rather than propulsive organs. The necks was longer and more flexible. Looks like the hands and feet had become paddles.

Those who suggested this was a filter-feeding bottom feeder were probably right on the money.

Look for more transitional taxa between these two in the future.

References
Cheng L, Chen XH,Shang QH and Wu XC 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaftendoi:10.1007/s00114-014-1148-4.